SHA-1 will still work fine for the purpose of git. It is just no longer considered secure for cryptographic operations, such as digital signature, that doesn't mean that you can't use it for other purposes, like git does. Using it is still fine and will ever be fine.
Making the hashing algorithm exchangeable would have introduces complexity in a software that is already complex, and also less efficient (one of the reasons git was created was speed for large project like the Linux kernel) for no real purpose. If you want to change the algorithm, given that you will break compatibility with all existing repositories, tools, and clients, you make a fork of git because you are changing too much.
I don't see why migrating to SHA-256. The collisions are still very unlikely to generate accidentally, and sure, if you want to do damage to a repository you can create one on purpose, as you can as well commit some hooks that contains malware, or alter the git history in any way you want, so what's the point?
Making the hashing algorithm exchangeable would have introduces complexity in a software that is already complex, and also less efficient (one of the reasons git was created was speed for large project like the Linux kernel) for no real purpose. If you want to change the algorithm, given that you will break compatibility with all existing repositories, tools, and clients, you make a fork of git because you are changing too much.
I don't see why migrating to SHA-256. The collisions are still very unlikely to generate accidentally, and sure, if you want to do damage to a repository you can create one on purpose, as you can as well commit some hooks that contains malware, or alter the git history in any way you want, so what's the point?